154 
EXPLORATIONS IN TURKESTAN, 
How much width of the Great Alai Valle\- was incUided in the block tilt is 
uncertain. It was, however, shown in the discussion on the Alai Valley fonn that 
there has been a lowering of base-level and consequent deepening of the valleys 
emptying into the great valley from the Trans- Alai range, and that this lowering 
had taken place since the close of the Alai glacial epoch. It was further shown 
that this took place with a depression of a belt including the side-valley systems from 
the north and possibly reaching to the ver>' base of the Trans-Alai range. The 
depression and filling up of the Sari Tash Valley and others running south from 
the Alai range was probabh- brought 
about by a block-tilt. This tilt can 
explain the observed changes of both 
the Alai and Trans-Alai ranges, and 
since the deepening of valleys in "-^ 
the latter took place after the close 
of the Alai glacial epoch, this tilt probably 
occurred since that epoch. It is, however, not 
likely that this one tilt was the only move- 
ment that took place ; in fact, we have seen 
that the border of the lowland plains was tilted 
up, presumably during the block-tilt, and there 
is no reason to suppose that the Trans-Alai 
range did not, at the same time, move either 
up or down a little on the block-tilt dislocation 
bordering it. If the tilting of the waste on the 
lowland plains was caused by a drag-up on the 
block-tilt dislocation there, the tilted waste was 
deposited largel>- during and before the Alai 
epoch, while the deposit overlying its lower portions are, in age, from bottom up, 
orogenic to present, inclusive. 
The relative antiquity of different horizons might be estimated in two ways: 
(i) as directlv proportional to magnitude of dip less normal surface slope of fans of 
that coarseness of material ; (2) as directly proportional to thickness of overlying 
accumulations. These methods would, however, be complicated from the fact that 
the rate of tilting seems to have been accelerated from the beginning, for the 
transition of valley form was always to the narrow from the broad. This compli- 
cation might be overcome by careful measurements of terraces and determination of 
the factor of acceleration. 
EX.\MPLE. 
Fig. 118. — Explanatory diagram for example in 
determining relative antiquity of horizons of 
the waste on the plains. 
Let n be the present surface with normal slope. 
the ages of which are to be detennined. (See fig. 1 1 
Let dA and dB = depths below surface. 
age ^ <{A- 
Let A and B be two horizons, 
S.) 
Let / = acceleration of tilting. 
estimate. 
ageB 
n) l_ 
<{B-n)^ f 
dA I 
—7- X -, as a rough 
